The present invention is related to optical fibers and, more particularly, to techniques for reducing reflection back into input optical fibers. Such techniques find applications in many fields, including optical devices such as isolators.
In present day optical fiber technology, semiconductor lasers are typically used to generate and relay light signals on optical fibers. These lasers are particularly susceptible to light signal reflections, which cause a laser to become unstable and noisy. Optical isolators are used to block these reflected signals from reaching the laser. Ideally, these optical isolators transmit all of the light signals in the forward direction and block all of the signals in the reverse direction. Thus an optical isolator's isolation, which is a measure of the amount of light lost travelling from an output fiber to an input fiber of the optical isolator, is much larger than its insertion loss, which is a measure of light lost travelling in the forward direction.
The present invention allows at least one type of optical isolator to raise its reflection loss near the ideal, i.e., that no light at all from the output fiber is transmitted back into the input fiber.
The central elements of this type of optical isolator are illustrated in FIG. 1A. The isolator has two birefringent crystal polarizers 10A and 10B, between which is placed a Faraday rotator 11. The rotator 11 is formed typically from doped garnet or YIG, and is placed in a permanent magnet 12. On one side of these central elements is placed the input optical fiber and on the other side is placed the output optical fiber. A collimating element, such as an ordinary lens or a graded index lens, is placed between each optical fiber and the central elements. (The optical fibers and collimating elements are not shown in this drawing.)
In the forward direction, collimated light from the input fiber is directed toward the slanted front face of the polarizer 10A. Each of the birefringent polarizers 10A and 10B have two indexes of refraction, one for the light polarized perpendicularly to the optical axis and another for the light polarized parallel to the optical axis of the birefringent polarizer 10A. The light, represented by a ray 14 in FIG. 1A, is split into two rays, an ordinary ray 14B polarized perpendicularly to the crystal's optical axis and an extraordinary ray 14A polarized parallel to the optical axis, in accordance with the polarization modes of the incoming light. The light from the polarizer 10A is rotated by the Faraday rotator by 45.degree.. Due to the orientation of the optical axis of the second polarizer 10B, the two rays 14A and 14B leave the second polarizer 10B in parallel and in a direction so that the second collimating element combines and refocuses the light into the core of the output fiber.
A slightly different operation occurs when light is sent back in the reverse direction, as illustrated in FIG. 1B. The light from the output fiber and second collimating element represented by a ray 15 is split into two rays 15A and 15B by the second polarizer 10B and rotated by the Faraday rotator 11. This rotation is nonreciprocal with the rotation of light in the forward direction, however, so that the ordinary ray 15B from the second polarizer 10B is polarized perpendicularly with the optical axis of the first polarizer 10A and the extraordinary ray 15A from the second polarizer 10B is polarized with the optical axis of the first polarizer 10A. The ordinary and extraordinary rays from the second polarizer 10B have swapped places incident upon the first polarizer 10A. Because of this exchange, the light, having passed through the first polarizer 10A, leaves the polarizer 10A in directions which are not parallel. The non-parallel light from the polarizer 10A is focused by the collimating element before the input fiber at points which are not located at the end of the input fiber. More precisely stated, the light is not focussed on the core of the input fiber and is theoretically not transmitted back into the fiber. A more detailed description of this type of optical isolator may be found in U.S. Pat. No. 5,208,876, entitled, "OPTICAL ISOLATOR," which issued May 4, 1993 to J. J. Pan.
Nonetheless, while the described optical isolator operates admirably with high reflecting losses, there is still room for improvement. The present invention achieves this improved performance so that an optical isolator can be built with low transmission loss and very high reflection loss.